This invention relates to a dielectric liquid for the machining of metal by means of spark erosion.
In the spark erosion process, an eroding machining of a workpiece takes place by spark discharges between a working electrode and the workpiece which acts as the other electrode. Between the electrodes, the dielectric fluid is situated which insulates the electrodes from one another in such a manner that the electric breakdown does not take place before the smallest possible distances are obtained. In addition, the dielectric must permit the buildup of a spark as fast as possible; it must constrict, if possible, the spark discharge channel during the discharge for achieving a high energy density; and it must rapidly deionize the discharge channel after the termination of the spark. In addition, it is used for the cooling of the working electrode and of the workpiece as well as for the carrying-away of the contaminants which are removed from the workpiece and, as a result of wear, from the working electrode.
The dielectric therefore has an important influence on the course of a spark discharge--a process which is not yet fully understood in all its details and which is the subject of intensive research. The different demands to be made on the dielectric fluid are described, for example, in the brochure "Important Facts About Spark Erosion" of the firm Oel-Held, March 1990 Edition. From this brochure, it is also known to use dielectrics on the basis of de-ionized water or of hydrocarbon compounds obtained by the distillation and refining of mineral oils or synthetically. Known dielectric fluids for the machining of metal contain either high concentrations of aromatic hydrocarbon compounds which are problematic with respect to occupational safety, or are relatively severely limited in their erosion performance as well as, particularly during polishing erosion, with respect to the surface quality because in this case the low energy contents of the respective used sparks have as a prerequisite an extremely narrow working gap which results in the collapse of the erosion process.
A transparent dielectric liquid of this type for spark erosion machines is known from European Patent Document EP 0 261 546 A2. This dielectric liquid consists of a mixture of mineral oil and/or a synthetic oil and a specific proportion of de-ionized water. In this case, paraffin oils, for example, are used as the mineral oil. Furthermore, a surface-active additive in a concentration of between 5 and 40 percent in weight is added to the described mixture, which additive has multiple compounds of carbon atoms. When such a dielectric is used, the above-mentioned disadvantages may occur.
German Patent Document DE 26 32 180 A1 discloses another dielectric fluid which consists, by at least 30 percent in weight, of aromatic compounds which contain at least two homocyclic, monocyclic C.sub.6 -ring structures. By means of such a dielectric, the different requirements during the roughing-down, on the one hand, and during the planing, on the other hand, are to be satisfactorily met in each case. However, from the viewpoint of occupational safety, the use of such aromatic compounds in such concentration is problematic. The use of such a dielectric therefore requires corresponding safety expenditures so that no toxic or allergic reactions will occur when this fluid is handled. It is known, for example, that a number of aromatic compounds belong to the cancer-causing substances. Dielectrics on a base of water or saturated aliphatic hydrocarbons do not contain such aromatic compounds in such high concentrations.
In British Patent Document GB-PS 10 03 664, the use of dielectrics is recommended which have a carbon:hydrogen ratio of between 0.8:1 and 1.8:1 and are obtained from a fractionation of coaltar. These are also hydrocarbon mixtures with a high proportion of aromatic compounds.
U.S. Pat. No. 3,639,275 describes the addition of an additive to a dielectric on a mineral oil base, the mineral oil containing between 10 and 30 percent in weight of aromatic hydrocarbons and the additive consisting of homocyclic monocyclic hydrocarbons in the form of phenol compounds as well as organic alkaline-earth sulfonates. This dielectric therefore also has a high proportion of aromatic compounds. Also, the addition of the phenol compounds which act as antioxidant agents presents problems with respect to occupational safety.
U.S. Pat. No. 3,648,013 sees an improvement of the characteristics of the dielectric by the addition of an alkyl salicylic acid chromium salt as well as an alkaline earth salt of certain dialkyl sulfosuccinates of a concentration of 1 to 10 percent in weight respectively for the purpose of improving conductivity. The alkyl salicylic acid chromium salt is a monocyclic aromatic compound with a heavy metal having a toxic effect, which today also presents problems with respect to occupational safety.
The adding of an additive to a dielectric on a silicone, oil or hydrocarbon base is also known from U.S. Pat. No. 3,708,422. There, the adding of aliphatic hydrocarbon amines is recommended which consist of 10 to 50 carbon atoms. By means of such an additive, the metal erosion index is to be improved, in which case the concentration of the additive is considered to be uncritical.
It is an object of the invention to provide a dielectric fluid of the initially mentioned type which, when it is used during spark erosion, causes a best-possible implementation of the process and particularly a high erosion performance while the wear of the working electrode is low at the same time and which can be used in a comparatively unproblematic manner with respect to occupational safety aspects.
This object is achieved by a dielectric fluid which contains saturated aliphatic hydrocarbons with a proportion of aromatic hydrocarbons which is lower than one percent in weight and to which an additive is added, wherein organic substances from the group of anthraquinone derivatives are added as the additive in a concentration of maximally one percent in weight. Since it consists essentially of a mixture of water and polyhydric alcohol or of saturated aliphatic hydrocarbon compounds which may have a naphthenic, n-paraffinic or iso-paraffinic structure or may consist of a mixture of these, no high concentrations of aromatic compounds will occur. At the same time, it was found that the addition of the additive of a higher polarizability in comparison to saturated aliphatic hydrocarbons in a concentration of maximally one percent in weight has a very targeted positive influence on the spark erosion operation. The addition of the additive to the dielectric base fluid takes place not only for the improvement of the metal erosion rate or for the reduction of the wear of the workpiece electrode. Its main purpose is to charge chemical satellite electrodes in the dielectric by means of substances which float in the dielectric fluid in a dispersed manner and, because of the effect of the electric field between the electrodes become stronger dipoles than the aliphatic hydrocarbons.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.